This invention relates to an electronic circuit for controlling various electrical apparatus installed in a vehicle equipped with an internal combustion engine, and more particularly to an electronic circuit of this kind which is provided with a fail safe function which is executed in the event of the occurrence of abnormality in the supply voltage.
Conventionally, there have been proposed a variety of control systems for controlling internal combustion engines for vehicles, such as air/fuel ratio control systems, fuel injection systems, automatic transmission systems, and exhaust emission control systems.
These control systems are each provided with an electronic control circuit, such as an electronic control unit (ECU), which includes a central processing unit (CPU). The electronic control circuit is arranged to control electrical apparatus which are installed in a vehicle for performing their respective intended operations, in response to operating condition of an engine installed in the vehicle.
For instance, according to an air/fuel ratio control system, which has been proposed by the assignee of the present application, an air/fuel ratio control valve is arranged in connection with fuel quantity adjusting means such as a carburetor, for controlling the quantity of air or fuel to be present in an air/fuel mixture being supplied to an internal combustion engine. An electronic control unit (ECU) is used to drive the air/fuel ratio control valve by means of an actuator such as a pulse motor which forms one of the above-mentioned electrical apparatus, to perform feedback control of the air/fuel ratio of the mixture to a predetermined value in response to an output signal produced by a sensor for detecting the concentration of an exhaust gas ingredient emitted from the engine.
Electronic control circuits used in or with various control systems inclusive of the above air/fuel ratio control system are usually formed of microcomputers. However, microcomputers in general are apt to have their operations affected by variations in the supply voltage. To cope with this, a microcomputer is connected to a voltage regulated power-supply circuit for supplying the same with a regulated supply voltage, to ensure normal operation of the component devices of the microcomputer so that the microcomputer can properly perform its required control operation.
Even with the above voltage regulated power-supply circuit, the possibility still exists that the supply voltage of the power-supply circuit becomes out of a predetermined tolerable range at the low-temperature start of the engine or in the event of a failure in the power-supply circuit per se or a break in the related wiring. On such occasion, the component devices of the electronic control circuit do not normally operate. If the control operation is then continued without taking any emergency measures, there can occur abnormality in the control operation, which can result in various disadvantages such as inferior driveability of the vehicle and increased amounts of exhaust gas ingredients emitted from the engine.